In recent years, a monitoring system configured to capture an image of a predetermined location using a surveillance camera, perform image processing of the captured image (a frame), and detect a moving body such as a human or an animal has been widely used in the field of security. In the monitoring system, an interruption has occurred such as an action of changing a direction of the surveillance camera and an action of covering an imaging lens with a cloth or a hand. When there is such an interruption, the surveillance camera is unable to continue normal monitoring. Therefore, a surveillance camera that determines whether an interruption has occurred and produces an alarm when an interruption has occurred has been proposed (for example, refer to Patent Literature 1).
The above-described surveillance camera generates a histogram showing a distribution of pixel values of a current frame and a previous frame, and performs normalization to match average values with such histograms. Therefore, the surveillance camera obtains a degree of similarity of shapes of a normalized histogram of the current frame and a normalized histogram of the previous frame, and detects whether an interruption has occurred based on a comparison result of the degree of similarity and a threshold value. The surveillance camera performs normalization to match average values to prevent a change in brightness from being erroneously determined as an interruption.